One field of application for such turbines is stirring gases or ventilation in ovens or similar installations used for performing physico-chemical treatments at high temperatures, the ambient medium being constituted, for example, by inert or non-reactive gases.
Usually, such turbines are made of metal, generally being built up of a plurality of elements assembled together by welding. The use of metal gives rise to several drawbacks. Thus, the high mass of the rotary parts requires large shaft lines and very powerful motors, and in any event sets a limit on speed of rotation. There is also a temperature limit because of the risk of the metal creeping.
In addition, the sensitivity of metal to thermal shock can give rise to cracks forming or to deformation. This unbalances the rotary mass, leading to a reduction in the lifetime of turbines and of their drive motors. Unfortunately, in the applications mentioned above, severe thermal shock may occur, particularly when massively injecting a cold gas in order to lower the temperature inside an oven quickly for the purpose of reducing the duration of treatment cycles.
In order to avoid the problems encountered with metals, other materials have already been proposed for making turbines, in particular thermostructural composite materials. These materials are generally constituted by a fiber reinforcing fabric, or "preform", which is densified by a matrix, and they are characterized by mechanical properties that make them suitable for constituting structural elements and by their capacity for conserving such properties up to high temperatures. For example, usual thermostructural composite materials are carbon-carbon (C--C) composites constituted by carbon fiber reinforcement and a carbon matrix, and ceramic matrix composites (CMCs) constituted by carbon or ceramic fiber reinforcements and a ceramic matrix.
Compared with metals, thermostructural composite materials have the essential advantages of much lower density and of much greater stability at high temperatures. The reduction in mass and the elimination of any risk of creep can make it possible to operate at high speeds of rotation, and thus at very high ventilation flow rates without requiring overdimensioned drive members. In addition, thermostructural composite materials present very great resistance to thermal shock.
Thermostructural composite materials therefore present considerable advantages with respect to performance, but use thereof is restricted because of their rather high cost. Other than the cost of the materials used, the cost comes essentially from the duration of densification cycles, and from the difficulties encountered in making fiber preforms, particularly when the parts to be manufactured are complex in shape, as is the case for turbines.